Welcome to the forum.
Actually the reason the fan comes on first is to make air flow to engage the sail switch. This happens to make sure the gas valve in the furnace does not turn on without airflow. The path where the fan blows has a flapper in it and when the air flow is strong enough it will make a complete circuit that allows the gas valve to open and allow gas to flow to the burner.
If your fan is not coming on then you need to find where you are losing the 12VDC at. Check the simple things first like the 12VDC fuse in the converter.
